Iron desulphurization



March 1, 1938. R. P. HEUER 2,110,067

IRON DESULPHURIZATION Filed Aug. 19, 1956 Sheets-Sheet 2 RMSQZHW/ 6e lleaer March 1, 1938.

R. P. HEUER 10,067

IRON DESULPHURIZATION Filed Aug. 19, 1956 5 Shets-Sheet 5 March 1, 1938. R R HEUER 2,110,067

IRON DESULPHURI ZATION Filed Aug. 19, 1936 5 Sheets-Sheet 5 A mow sLAG (MEDIuM SULPHUR) (HIGH 31mm}? mm: SLAG (man SULPHUR) -OARBON AIR OR OXYGEN oxlmzl'ut; .suL HuR IRON l or mourzu sLAG (MEDIUM SULPHUR) SULPHUR DIOXIDE .SLAG (LOW SULPHUR) CARBON I LADLE 5M6 (M IRON smuu SULPHUR T -1 SULPHURD 6?. mom

3M6 (MEIHUM SULPHUR) (HEHSULHW .SLAG (man SULPHUR) CARBON REDUClNG TO POWDER mow RoAs-rmG (MEDIUM v SULPHUR) am. PHUR DIOXIDE msurms mn REACTION suwuun .mqxm:

CARBON sue (LOW suwnuny )"fl/ IRON a. (MEDIUM suLPuuR) V I}; 6?

- RON SLAG mznmM SULPHUR) 596 (HIGH SULPHUR) MATTE FORMING mew.

LOW m SULPHUR i MILD onDATION mo MATTE Rmwms METAL (mflm man m SULPHUR I SULPHUR) \smous oxuumom SLAG (Law sumuun) :M-PWR mum:

, cannon 'LAIJLE) mos sue (manwm sumnun) wwsm.mun

Patented Mar. 1, 1938 UNlTED (STATES PATENT. OFFICE IRON DESULPHURIZATION Russell P. Heuer, Bryn Mawr, Pa.

Application August 19,

' 23 Claims.

phur than that; ultimately desired, and preferably at a lower cost than normal, by operating the blast furnace at a lower temperature and/ or with less ba'sic slagand/or with low grade raw materials causing higher sulphur in the charge, and to treat the molten pig iron thus produced with a basic slag external to the blast furnace hearth, subjecting the slag and molten pig iron together to a pressure less than normal atmospheric pressure to remove the excess sulphur from the molten pig iron. The pig iron thus produced may be used in the form of cast iron or as raw material for making steel.

' A further purpose is to agitate molten pig iron during desulphurizing treatment with a basic slag under reducing conditions and with the exclusion of oxidizing substances while subjecting the pig iron and slag to a pressure less than normal atmospheric pressure.

A further purpose is to desulphurize molten pig iron excessively high in sulphur by treating it in a closed vessel with a basic desulphurizing slag in the presence oi an'excess of a noncontaminating reducing gas such as carbon monoxide under a pressure less than normal atmospheric pressure.

' A i'urther purpose is to build up a higher ratio between the sulphur in the slag and the sulphur in the pig iron than would be obtained with the same slag under atmospheric pressure and under substantially equilibrium conditions.

A further purpose is to desulphurize by a slag which would not be desulphurizing at the same temperature under atmospheric pressure.

A'further purpose is todesulphurize an, iron bath under vacuum conditions and in the presence of a slag and to subsequently protect against resulphurization of the bath during removal of r the slag by filling the space above the bath by a. non-contaminating gas at atmospheric pressure. A further purpose is to prevent resulphurization of a bath of molteniron which has been freed from sulphur under vacuum conditions, by filling the vacuum space with gas free from contamination by oxygen, carbon dioxide, carbon monozide and water vapor. v

A further purpose is to economize upon the use of desulphurizing slag in any of the processes disclosed herein by eilecting the desulphurizing treatments in stepwise manner, preferab y 11 1936, Serial No. 96.743

tially subjecting the pig iron to a first desulphurizing slag which is relatively high in sulphur content and therefore limited in its ability to take up sulphur from the pig iron, and, after removal of the first slag, treating the pig iron with one or more further slags of lower sulphur content and correspondingly'greater ability tov remove sulphur. In this 'way, pig iron containing substantially more than 0.10% of sulphur, say 0.3%

of sulphur or more, can have its sulphur content reduced to less than 0.05% sulphur, that is, to less than half, by treatment for an hour or more with slags which aggregate less than 8%, for example, only about 5%, of the weight of the pig iron. 7

A further-purpose is to desulphurize 'molten pig iron with a slag capable of absorbing sulphur I up to a percentage at least one hundred times the percentage present in the desulphurized iron, to maintain the slag and molten pig iron under a pressure less than normal atmospheric pressure, to separate the slag from the desulphurizing vessel and from the molten pig iron and to revivify the slag while it is out of contact with the molten pig iron to permit the reuse of the slag for desulphurizing more iron. v

A further purpose is to withdraw molten pig iron and molten blast furnace slag from the blast furnace and to desulphurize the molten'pig iron by the molten blast furnace slag under reducing conditions in a vessel maintained at a pressure below normal atmospheric pressure by a vacuum pump or the like. 1

A further purpose is to desulphurize molten pig iron in a stepwise manner in a closed vessel under a pressureless than normal atmospheric pressure by initially treating it with a first slag relatively high in sulphur which. has been used to desulphurize a previous charge of pig iron and not since revivlfied and by subsequently treating it with a second slag which is relatively low in sulphur, revivifying each slag out of contact 'with the pig iron when its sulphur content becomes high enough to prevent eillcient desulphurlzationr In normal operation, the first slag after use is immediately revivified and the second slag after use is preserved for treatment of the next pig iron charge before revivification.

A further purpose is to desulphurize molten pig iron by a slag in such manner as to protect the slag from the influence of free oxygen, predominantly oxidizing oxides of carbon and water during the desulphurizing step whilst maintaining the molten pig iron and slag under a pressure le s han normal atmospheric presure, to subsequently treat the sulphur-bearing slag in the presence of carbon dioxide and water to remove its sulphur content and to use the treated slag to desulphurize additional pig iron.

A further purpose is to desulphurize molten pig iron by a slag in an enclosed vessel provided with means to maintain the pig iron and slag under a pressure less than normal atmospheric pressure and desirably provided with means for agitating the molten contents of the vessel.

A further purpose is to use the pig iron desulphurized in the novel manner disclosed herein for the manufacture of steel, preferably as a continuous process in which the iron remains molten from the time it leaves the blast furnace until the time that it leaves the steel-making furnace.

Further purposes appear in the specification" and in the claims.

The invention relates not only to the process involved, but also to the slag employed and to the apparatus used and is in part a further development and improvement of the inventions described in my copending United States patent application, Serial No. 20,555, filed May 9, 1935.

ciation of iron sulphide. The abscissae are ab-' solute temperatures.

Figure 2 is a diagrammatic side elevation of a desulphurizing vessel which may be used in practicing the invention.

Figure 3 is a left end elevation of the structure of Figure 2.

Figure 4 is a transverse section of Figure 2 upon the line l-l thereof, with the top opening free for admittance of material and a furnace and runner in place.

Figures 5 and 5a. are fragmentary views corresponding to a portion of Figure 4 respectively without and with permissible heating electrodes and showing the opening 36 closed by an airtight cover.

Figure 6 is a chart of a desirable process of desulphurizing pig iron and revivifying the slag in accordance with the invention.

Figures 6a, 6b and 6c are charts of various parts, and in the specification like symbols refer to like subjects matter.

Throughout the specification, wherever reference is made to pig iron, it is intended to designace in such a manner as to obtain strong desulphurizing conditions in the hearth and bosh of the blast furnace. As a result the operation of the charcoal blast furnace is quite economical except for the excessive costs of charcoal and of low sulphur ores. In most instances, however, because of the high cost of wood charcoal, it is necessary to substitute coke for wood charcoal as a blast furnace fuel. When this is done, a strong desulphurizing action must be obtained in the hearth and bosh of the blast furnace in or- A typical slag'from a charcoal furnace approximates H I Per cent S10: --LJLL' 47.0 A1203 18.0 (39.0 27.0 MgO 3.0 S 0.10

In the above slag the silica exceeds the lime plus magnesia. The sulphur in the pig iron approximates 0.02%.

A typical slag from a coke blast furnace has the following approximate composition Per cent S10: 34.0 A120: 12.0 CaO 40.0 MgO 10.0 S 1.7

In the above slag the silica is less than the lime plus magnesia. The sulphur in the pig iron approximates 0.03%.

The temperatures of the slag and pig iron tapped from a charcoal blast furnace are about 1450 C. and 1410" C. respectively. In the coke blast furnace, the respective temperatures of the slag and pig iron as tapped are approximately 1525 C. and 1475 C. Thus it is unnecessary for the charcoal blast furnace to expend as much heat to produce slag and pig iron as does the coke blast furnace.

From the above data it will be obvious that the cost of removing sulphur from the pig iron simultaneously with smelting of the ore in a blast furnace is considerable. It is one purpose of the present invention to cheapen the cost of manufacturing pig iron, whether for use as cast iron or for making steel, by omitting the strong desulphurizing action now considered necessary in the coke blast furnace because of the sulphurbearing burdens. The iron is smelted in the coke blast furnace underconditions similarto those prevailing in charcoal blast furnaces, desirably using a blast furnace slag in which the silica ex-' ceeds the lime plus magnesia. The smelting is conducted in the coke blast furnace under conditions of temperature and basicity of slag characteristic of the charcoal blast furnace and suitable to produce pig iron at lower cost but with a higher sulphur content in the iron than that ultimately desired. The sulphur content in the iron produced may be for example 0.25% sulphur. After production of this high sulphur pig iron, it is treated in novel manner as a wholly independent operation to remove the excess of sulphur. The cost of the desulphurization is small and it is possible to produce pig iron of desirable quality at a considerable saving. Of course it is cium oxide, barium oxide, strontium oxide, sodinot necessary that the high sulphur pig iron contain as miich as 0.25% sulphur before treatment. The process is equally adapted to the desulphurization of pig iron'containing 0.10% sulphur or 0.05% sulphur or less depending upon individual conditions. i

The pig iron, containing a higher sulphur content than that ultimately desired, is transferred from the coke blast furnace to a suitable desul-' phurizing vessel as described later. The pig iron is there subjected to a strongly basic slag under reducing conditions while subjecting the iron and slag to 'a pressure less than normal atmos- Due to this dissociation, there is a certain activity of sulphur in the molten pig iron which is measured at constant temperature by the equilibrium constant for Equation (1) thus rmrwhere (Ar|)=activity of iron (A activity of sulphur (A activity of iron sulphide K =equilibrium constant The iron sulphide is considered to be present in solution in the molten pig iron and its activity varies as the concentration. The relation of sulphur activity and sulphur concentration is expressed in Figure 1 taken from H. Schenck, Physikalische Chemie der Eisenhiittenprozesse (Verlag von Julius Springer, Berlin, 1932) -vollime 1, page 263. In this figure, the ordinates are square roots of the activities of sulphur from dissociation of iron sulphide and the abscissae are temperature in degrees absolute (K The curves are plotted for an iron-sulphur system free from carbon.

Within the area D C E F G H 1, iron sulphide and iron occur as liquid solutions. In these solutions the sulphur activity ,is a function of the sulphur concentration. In Figure 1, diagonal dotted lines are shown corresponding to the sulphur percentages indicated by the numerals to the right of the dotted lines. The sulphur activity resulting from a given sulphur percentage in the melt and a given temperature is shown by these dotted lines. It would require a third dimension to show the relation of sulphur activity, sulphur concentration and temperature for liquids containing iron, iron sulphide and carbon. As an approximation for example to conditions prevailing in molten pig iron of 0.02% sulphur content at 1400" C. (16739 K.) we may extend the line for 0.02% sulphur outside the area D C E F G H I and into the area B C E F G until the temperature l673 K. is reached and learn that at J the equals approximately lfor 0.02% sulphur at sulphur activity, the sulphur content will fall.

The desulphurizing slag contains basic oxides of the alkaline earth or alkali metalssuch as caliron thus:

um oxide, etc. These are referred to generally as R oxides, where R. is an alkaline earth or alkali metal and a: is the numeral 2 or 1, depending upon the valence of the metal. The oxides of the slag tend to react with sulphur of the pig 3.0+ asl=ms+aoi (a) For this reaction the equilibrium at constant temperature is expressed thus:

. (Ag o)=activity of R,O

(A )=activity of R,S

( 0,)=activity of oxygen K; equilibrium constant cage, co 5 The equilibrium expression for this reaction is mam (6) where (Ac) ==activity of carbon (Ace) =activity of carbon monoxide K3=equilibrium constant where AFo=free energy change R=gas constant T=absolute temperature Zn=logarithm to the base e The free energy change at a given temperature is expressed by the following equation, taken from International Critical Tables, volume VII,

page 243:

AF 26,600 2.15TInT+ 0.00215T 0.000G002T 8.20T

At 1673 K. (1400 C.) the .free energy change I AFo: 61,398

assuming the activity of both carbon and carbon monoxide to be unity. In the present invention it is desired that the activity of carbon monoxide shall be less than unity and this desirable condition is obtained by diminishing the Cilpressure of the reacting pig iron and slag below the normal atmospheric pressure of 760 millimeters of mercury. This pressure may be reduced, for example, to approximately 78 millimeters of mercury, thereby reducing the activity of carbon monoxide to approximately one-tenth of its former value. By so doing the former value of as calculated from Equations (6) and (7) is reduced from 8.2x10- to 8.2xl0- If, for the purpose of illustration, it be assumed as in United States patent application Serial No. 20,555. that it is desired to desulphurize pig iron until the sulphur activity reaches a point at which s- Equation (4) may be solved using this value of B and the previously determined value of (A =s.2 x 10- for 760 millimeters of mercury pressure, thus: (AR,s 3.9 1on a) X 1,o) 2

In the present invention, using the illustration previously described for '16 millimeters of mercury pressure'where (Ao,)%=s.2 1o- Equation (11) becomes n s iz m u oxz The value of m-s) is a measure of the concentration or percentage of RXS in the desulphurl-zing slag. From Equation (12) it is evident that as the pressure on the slag and pig iron falls below the normal atmospheric pressure of 760 millimeters of mercury, the value of rises proportionally. Thus for a pressure of 76 millimeters of mercury, the lowering of pressure to one tenth normal atmospheric pressure causes high through choice of a slag of proper chemical composition, and by keeping the pressure on the slag as far below atmospheric pressure as is practical and economical. At this point it is interestlng to note that the conditions attending the I desulphurization of pig iron in the blast furnace hearth are not particularly favorable because the pressure is as much as 760 millimeters of mercury or more above atmospheric pressure.

Slugs .ever, that CaO is a very desirable and economical desulphurizing agent and it is used in the illustrations, but it will be evident that the other oxides described above could be used in a similar manner after proper allowance is made for slight variations in the chemical and physical properties of the other oxides.

CaO in the pure state is not suitable physically for forming a desulphurizing slag dueto its high melting point (2570 C.) 'It is conventional to reduce the melting point of CaO to form readily fusible slags by adding SiOz and perhaps A: and other oxides. Thus, for example, it is possible to form a slag which is free-flowing at 1400" C. having the approximate composition:

' Percent CaO 44 MgO 4 A1203 "15 S10: 3'7

In such a slag the activity of the lime is reduced by the presence of 37% silica and 15% alumina. Slags of this approximate composition are used as desulphurizing agents in the blast furnace and are capable of building up a 60:1 ratio of the percentage of sulphur in the slag to the percentage of sulphur in the pig iron; It has been found that such a slag can be used in the present invention, especially if the slag and pig iron to be desulphurized are subjected to a diminished pressure of millimeters of mercury or less, thereby causing the sulphur ratio to rise to 100:1 or 200:1 or more depending upon specific conditions.

The foregoing slag may also be adjusted to contain higher or lower silica contents. .In general, as the silica content falls the sulphur ratio rises but the minimum free-flowing temperature of the slag rises also. Thus it becomes necessary to add agents such as CaFz (fluorspar) to make the slag flow properly at operating temperatures. It has been found that the addition of fiuorspar in amounts of 5% or 10% or more to slags containing about 45% to 50% CaO and 35% SiOz is satisfactory. The SiOz' content can also be decreased and slags have been synthesized containing as little as 15% $102 or less, and it has been found that such slags are very advantageous especially where the slag and pig iron under treatment cannot be subjected to low pressures.

For example a desulphurizing slag having the composition:

. Percent CaO 45 CaFz 40 S102 15 has been found to be very satisfactory. Such a slag can be readily synthesized by mixing lime and fluorspar of commercial grade, adjusting the silica to the desired percentage by the addition of small amount of impurity is also not objectionable. Indeed the addition of approximately 5% of alumina seems to be advantageous as it reduces somewhat the melting point of the desulphurizing slag. The above slag flows freely at 1200 C. and this property is very desirable for efficient desidphurization.

Molten pig iron high in sulphur (0.25% S) has been subjected to the action of the limefluorspar-silica slag above referred to at 1400 C. for one hour at atmospheric pressure in a refractory container composed of carbon in the form of graphite. 'Ihe slag and molten pig iron were under strong reducing conditions and the desulphurizing vessel was arranged so as to prevent direct contact with the atmosphere or with combustion gases high in oxygen, carbon dioxide or water. Under these conditions the slag picked up as much as 10.79% sulphur, and the pig iron had its sulphur content reduced to 0.03% sulphur,. so that there was more than one hundred times as high a percentage of sulphur in the slag as in the pig iron. In other cases in which the pig iron initially contained only about 0.03% sulphur,

the sulphur content of the pig iron was reduced to 0.002% sulphur and the slag picked up 0.11%- sulphur.

These good desulphurizin g results can be still further improved if the pressure on the slag is reduced, for example to 380 millimeters of mercury, or below. It is to be noted, however, that low pressures such as 30 millimeters of mercury are not to be recommended with slags rich in fluorspar unless adequate means are provided to take care of the volatile products which such slags give 011 at low pressures. The sulphur content of the iron may be reduced to less than onehalf, or less than one-quarter, or even less than one-tenth of that in the iron before treatment by the invention.

Astudy has been made of the useof soda as a substitute for lime in desulphurizing slags. The desulphurizing actionof soda, for example so- .dlum carbonate, on pig iron is well known and this substitution can be made. It is found, however,

that pure soda is readily attacked by carbon at.

temperatures of 1400 C. or even below, producing sodium vaporwhich is very effective in converting FeS intoNazS and thus desulphurizing the pig iron. The sodium vapor causes difficulty in the handling of soda slags, and the reaction must be managed in such a way as to limit the formation of volatile soda derivatives in order to avoid excessive loss as fume. It is possible to use slags containing NazO, S02 and A120: with other oxides to overcomefvolatilization loss incident to sodium carbonate. Furthermore the revivification of soda slags to remove sulphur and permit reuse presents special difficulties not present in the case of lime slags.

Detailed reference to the steps necessary when soda slags are used is therefore omitted, and the further discussion is generally confined to slags containing alkaline earth oxides as the preponderant active constituent. However, the use of slags containing soda or other alkali metal oxides as active desulphurizing ingredients, when used as substitutes for alkaline earth oxides such as lime as'active ingredients, is claimed herein.

. The quantity of oxides'of the type RrO in the initial slag, should exceed 30% for best results.

Dueto the cheapness and freedom from volatilization, it is preferable to use a lime slag, although such preference is subject to change under varyingeconomic and metallurgical conditions.

In choosing'the slag, it is desirable to have one which will be of low viscosity and workable at temperatures of 1400" 0., and, for this purpose,

the. slag should-preferably be freely fluid as low as 1200 C. or in some cases at 1300 C.

It is very advantageous to use a slag which shows a high ratio of sulphur concentration in the slag after use to sulphur concentration in the desulphurized or partially desulphurized pig iron. If, for example, 0.25% sulphur must be removed from the pig iron and if the slag will take up as much as 12.5% sulphur, then one ton of slag will desulphurize 50 tons of pig iron. It is to be noted that an ordinary coke blast furnace slag can absorb approximately 1.75% sulphur and reduce pig iron to as low as 0.03% sulphur under a pressure as much as 760 millimeters of mercury above atmospheric. It has been found that such an ordinary blast furnace slag when subjected under proper conditions to approxi- -mately one tenth normal atmospheric pressure (for example 76 millimeters of mercury pressure), becomescapable of absorbing 6% of sulphur or more and can. thereby reduce a high sulphur iron to 0.03% or less. In such a case the ratio of sulphur percentage in the slag to sulphur percentage in the molten pig iron equals or exceeds 200:1.

Thus by the use of diminished pressure it is possible to improve the desulphurizing power of the special synthetic slags such as the one described above containing 45% Geo, 40% Cal- '2, 15% S102, or to improve the desulphurizing power of ordinary blast furnace siags or similar slags of poorer desulphurizing ability to the point where they may be substituted for the special synthetic slags. If slags produced in a blast furnace containing about S102 and of lime are to be used, the addition of approximately'5% to 10% of fluorspar is advantageous.

It is obvious that the composition of the slag and the diminished pressure to be used should be chosen to meet the individual case under consideration. Much latitude is offered in the choice of these two factors as is shown by Equations (4) and (12). In general these factors of slag composition and pressure should be chosen so that a ratio of sulphur in the slag to sulphur in the iron should exceed 100:1 and preferably exceed 200:1.

As explained in detail below, it is often advantageous to employ a desulphurizing slag which can be revivified or treated to remove its sulphur so that it can be used over and over again. The lime slags referred to fulfill this requirement. The desulphurizing slag, after it has picked up, for example, 12% sulphur, is revivifled by removing sulphur from the slag until, for example, less than 1% sulphur remains. This revivified slag is then used repeatedly to desulphurize further quantities of molten pig iron.

Desulphurization The pig iron used in the process will in most cases be pig iron produced in the coke blast furnace when operated under conditions of moderate temperature and moderate basicity of the slag suitable to produce pig iron at lower cost but with a higher sulphur content than that ultimately desired. The sulphur content of the pig iron produced in the coke blast furnace under such economical conditions may be 0.25%, 0.3% or even higher. Of course the invention may also be applied to pig iron of normal sulphur content, containing say 0.04% S. The pig iron is tapped from the coke blast furnace, and, ordinarily, separated from the,blast furnace slag at the time of tapping. Where the blast furnace slag is to accomplish the desulphurizing, the separation of the pig iron from the blast furnace slag at the time of tapping is not necessary as far as the present invention is concerned. The pig iron is then preferably brought at once into contact with the desulphurizing slag, while the pig iron is still molten from the blast furnace.

The actual desulphurization may, for example, be carried out in a vessel such as that shown in Figures 2to 5, inclusive, which illustrate a ladle car of the general type disclosed in Pugh 'United States Patent No, 1,534,187, granted April 21, 1925. The ladle car comprises a ladle body 20, covered with a metallic casing 2| having a cylindrical central portion 22 and conical end portions 23 and 24 which terminate in headers 25 and 26. The headers support trunnions 21, 21', 21 and 21 which engage bearings 28, 28, 28 and 28 The bearings 28, 28', 28 and 2B are supported from a main frame 29, which in turn rests upon any suitable railway trucks 30 and 3|, operating upon a track 32.

To permit tilting or rocking of the ladle car,

the main frame 29 is bowed at 33. The hook of a crane may be engaged with the main frame, as at 34, to lift one side of the main frame, causing trunnions 21' to leave bearings 28 and eventually causing trunnions 21 to engage bearings 28 Lifting and lowering at 34' might also be used. The lifting and lowering of one side of the main frame may be used to agitate the liquid contents of the ladle car,and the lifting and lowering at 34 may also be used in tapping the ladle car.

The means of rocking and tilting the ladle car need not be that shown, as any other suitable means may be employed. For example, the reaction vessel of the ladle car may be rotated by a conventional driving band and motor as shown, for example, in Hart United States Patent No. 1,916,170, granted June 27, 1933.

Inside the casing 2| is a lining 35 of suitable refractory material. It is contemplated that this will normally be graphite, or other suitable carbon refractory, although any other suitable lining material, such as magnesite, for example, might be used. If carbon be used as a lining, it may be rammed in place with a tar binder or built into the desired form from blocks which have previously been fired.

Inlet to the ladle car is provided through a charging opening 36, from a runner or launder 31 of a coke blast furnace 38. The charging opening may be closed by a door 39 which makes a. gas-tight seal through gasket 40 between flanges H and 42.

The discharge of slag from the ladle car is facilitated by a pouring spout 43 below the charging inlet. When the pouring spout is used, the ladle car will be tilted, and the pouring spout will normally be closed when not in use by a plug 44 and a door 45 making a gas-tight seal through gasket 46 between flanges 41 and 48. Gas-tight inspection doors may be provided if desired. The ladle car is provided with a connection 49 leading to a suitable pump 49' for maintaining the desired pressure below the normal atmospheric pressure.

molten desulphurizing slag,

An atmosphere of reducing gas, primarily carbon monoxide, under a pressure below atmospheric pressure is maintained in the vessel by the pumps. The carbon monoxide comes from the reaction of any oxidizing substances with carbon present. In order to maintain this pressure the entire steel shell of the ladle and all openings are made ,gas tight.

It is not ordinarily-necessary to heat the ladle car, as the pig iron will normally have sufficient superheat from the blast furnace to keep the contents of the desulphurizing vessel molten and to melt the desulphurizing slag when it is not charged molten. For the purpose of illustrating diagrammatically that any suitable noncontaminating heating means may be applied to the ladle car, electrodes 50 are shown in Figure So (one set only being visible in this figure) intended to be connected to a suitable source of electrical energy to supply are heating. No attempt in the illustration-has been made to show either electrical insulation or protection against air leakage about or for the electrodes as both insulation and closure against leakage may be taken care of in various well recognized ways. In fact the leakage incident to the electrodes need not be taken care of as the electrodes and the covers carrying them may be replaced by gastight doors after heating and before applying vacuum.

7 An illustrative cycle of operations is indicated 'in Figure 6. The specific values 'given on this figure vary somewhat from those mentioned in other examples, and are given as a particular instance of the process, without intention to limit the disclosure.

The ladle car containing about 2.5 tons of preferably slag which has previously been used to desulphurize a previous charge of molten pig iron, is filled with about tons of molten pig iron from the coke blast furnace flowing through the blast furnace runner 31, and containing as much as 0.3% sulphur or more (say 0.26% sulphur). The

molten slag is held in contact with the molten iron under the proper diminished pressure for as much as one hour or more until the sulphur content of the molten pig iron has fallen to between about 0.03% sulphur and 0.10% sulphur (say 0.05% sulphur) and the sulphur in the slag has increased to perhaps 5% sulphur or a much higher figure (say 10.8% sulphur). With good operating conditions and eflicient slags, as much as 12% sulphur can be built up in the slag with only 0.04% or 0.05% sulphur in the pig iron. It is desirable to have a high concentration of sulphur in the slag when the slag is to be revivlfied since the amount of slag necessary is thereby reduced.

At this stage the slag is then removed from the ladle car by the pouring spout 43.

In the preferred process about 2.5 tons of a second slag similar in composition to the first slag are now added, and further desulphurization of the molten pig iron is accomplished.

The pig iron and slag are maintained in contact with one another preferably for an additional hour or more under the proper diminished pressure, and the sulphur content of the pig iron may thereby be reduced to 0.015% sulphur, 0.01% sulphur or even less if desired. The

ond slag,'as by tapping the second slag, then removing the molten iron, and then pouring back the second slag into the ladle car, or by retaining the molten slag in the ladle car duri tapping of the iron, for example by submerging the pouring opening below the slag level before removing the plug from the pouring opening and then retaining the slag level above the pouring opening during pouring from the ladle car.

The ladle car ls then returned to a source of high sulphur pig iron to receive a further charge of say 100 tons, and the further charge is desulphurized by a first treatment with the slag which was used as the second slag on the previous P charge, by removal of the high sulphur slag produoed thereby, by addition of fresh slag and so on.

It will of course be understood that a stationary ladle, mixer or furnace can be employed, if it is suitably equipped to maintain sufllciently reducing conditions and suitably low pressures.

The amount of slag required per ton of pig iron desulphurized will depend on the amount of sulphur to be removed from the pig iron and the amount of sulphur picked up by the slag. If, for example, a high sulphur pig iron containing 0.26% sulphur is desulphurized to 0.015% sulphur, then, for every 100 tons of pig iron, 0.245 tons of sulphur must be taken up by the slag. If the slag picks up 9.8% sulphur, 2.5 tons of slag will be necessary to treat 100 tons of pig iron.

In desulphurizing high sulphur pig iron, a stepwise process as described above is quite eilicient since it removes the sulphur with a small amount of slag, and necessitates regeneration of a smaller amount of slag for further desulphurization. For efllcient desulphurization the ratio of sulphur percentage in the slag to sulphur percentage in the pig iron may be as much as 250:1 or even a higher ratio. Thus a desulphurizing slag in contact with molten pig iron having 0.015% sulphur might have picked up 2.5% sulphur from the pig iron.

If 100 tons of iron were being treated to remove 0.25 ton of sulphur, 10 tons of slag might be needed if the desulphurization were done in a single step.

If the pig iron were desulphurized in a stepwise process as shown in Figure 6, reducing the sulphur first to about 0.05% sulphur and separating a sulphur-rich slag containing 10.8% sulphur, then adding a second and fresh slag to remove the balance of the sulphur in the iron iii) down to 0.015% sulphur, then separating the iron from the second slag containing about 2.4% sul-, phur for use in the preliminary treatment of the next charge of sulphur-rich pig iron as above described, the removal of 0.245 ton of sulphur from 100 tons of iron can be accomplished with only 2.5 tons of slag, although 3.5 tons of slag. or more, may be used.

By stepwise treatment it is possible to reduce the sulphur content of molten pig iron from a value in excess of 0.10%, say 0.3%, to less than In the'case oi a lime slag, sulphur is retained in the slag as calcium sulphide. Iron sulphide present in the iron phase dissociates:

FeS- Fe+ ass, (1)

The sulphur reacts with the lime or other R10 oxide of the slag thus:

Ca0+ %Sz- Cas+ Ago: (13) To drive the above reaction to the right and desulphurize the pig iron it is necessary to keep the oxygen activity low by reducing agents. The

high carbon content of the pig iron itself has some reducing action, and it is decidedly preferable to desulphurize the iron while it still has its pig iron carbon content to aid in reduction then at a later stage when its carbon content has been lowered, for example to that of steel. In many cases the carbon content of the pig iron itself will serve to maintain reducing conditions when air, carbon monoxide and water are excluded.

It is desirable to employ a carbon lining in the ladle car, and this serves to assist materially in maintaining reducing conditions. It is also very desirable where a carbon lining is used, and much more desirable where a carbon lining is not used.

to introduce coke or charcoal into the slag, mainwater vapor-for example nitrogen. If a gas, so

free from contamination, be admitted under atmospheric pressure to fill up the space, resulphurization of the pig iron will be prevented during the short interval while the slag is being separated from the iron.

If other reducing agents than carbon be desired to remove oxygen in reaction (13), calcium carbide, ferrosilicon, or other reducing ferro alloys may be added to the slag. The lower the oxygen activity, the higher the possible ratio of sulphur in the slag to sulphur in the pig iron at the end of desulphurization, the more complete the desulphurization and the more rapid the reaction.-

To facilitate the desulphurization, agitation of the metal and slag may be used. Such agitation may be produced by rocking or rotating the ladle car, by raising and lowering one end of the ladle car, etc.

The desulphurized pig iron from the ladle car may be used in any form in which the pig iron is suitably employed, as for example for gray iron or malleableized castings, etc. It is contemplated, however, that a large part of the desulphurized pig iron will be used in steel-making furnaces for the production of steel in much the same manner that the conventional low sulphur product of the coke blast furnace is now used.

Where steel is made, the economy in production of the raw material will effect an over-all economy in the steel process. A further important advantage in steel making is that it will in no case be necessary to take any precautions in steel making to eliminate sulphur, as is some-' times necessary when the blast furnace pig iron runs excessively high in sulphur. This is of es- Revivification It has been previously explained that a wide choice of desulphurizing slags is accorded in this invention, and that special slags containing high percentages of. fluorspar or other expensive components may be used.

To utilize these expensive slags economically the inventor proposes 'to revivify the spent slags and use them over and over again. In many cases, however, a special slag may not be required.

For example, let us consider the desulphurization of a pig iron containing about 0.10% 8. One hundred tons of such pig iron could be charged into the mixer ladle with approximately 4.5 tons of molten slag taken simultaneously from the blast furnace. I! desired 0.5 ton of fluorspar could be charged. The slag and pig iron could be maintained for example under a pressure oi '76 millimeters of mercury for one hour ormo're. Under such conditions the sulphur in the iron would be reduced to less than 0.025% (for example to 0.02%). Such reduction would require the 5.0 tons of initial slag to absorb 0.08% of 100 tons or 0.08 ton of sulphur, the equivalent of 1.6% sulphur in the slag.

Since the initial slag above was obtained from a blast furnace it may have contained initially about 1.5% sulphur. Thus the final slag would contain 1.5% plus 1.6% or 3.1% sulphur. This condition would require a ratio of the percentage of sulphur in the slag to the percentage oi sulphur in the iron of 155:1. Ai'ter desulphurization, the molten iron could be tapped from the ladle before the slag or the slag could be'separated first. In separating slag and metal it is desirable to avoid any excessive oxidation whilst the slag and metal are in contact since this will cause sulphur to leave the slag and reenter the metal. Similarly, .after the desulphurization is completed it is desirable to separate slag and metal as soon as possible after the pressure has i .been restored to normal atmospheric pressure.

In this last illustration the desulphurizing slag need not be revivifled for use over again since fresh quantities of molten blast furnace slag could be obtained more cheaply than the used slag could be revivifled. The choice as to whether special slags should be used or whether the cheaper blast furnace or similar slags will suflice will depend upon individual conditions, the sulphur contents of the initial and final iron, the

cost of revivification, etc.

There are several ways of revivifying the de- I v desulphurization is impeded, and with addition or fresh slag when the losses of slag necessitate augmenting the quantity. It will be noted that, in accordance with the invention, the conditions are always reducing when the desulphurizing slag is in contact with the molten pig iron, thus minimizing the picking up of phosphorus, manganese, etc., by the slag. If the slag were in contact with the iron when oxidation was possible, pick-up oi phosphorus, manganese, etc., by the slag 'wouldbe excessive. Naturally, a substantial pick-up of phosphorus, for example, would render the slag unusable even though it were revivified to lower its sulphur content, and therefore it is important-to avoid having contact between the moltenpig iron and the desulphurizing slag under unsuitable oxidizing conditions.

The slag is revi'viiied while it is out or contact with the molten-pig iron and preferably after separation from the desulphurizing vessel, as reviviilcation involves oxidizing reactions which would be harmful to desulphurization.

It is preferred to effect the reviviflcation by the principles underlying the process of Claus and Chance as used for removing the sulphur from calcium sulphide present in tank wastes produced in the LeBlanc soda process. See George Lunge, Sulphuric acid and alkali (D..Van Nostrand Co., N. Y., 1909) volume 2, part 2, page 943 et seq. This reviviflcation process applies to slags containing any oi the alkaline earth sulphides, but calcium sulphide, being the cheapest. is referred to below in the specific example. In accordance with this process the desulphurizing slag containing perhaps 35% oi calcium sulphide is reduced to a fine powder, preferably to a state of subdivision such that it will pass through a 50 mesh per linear inch screen. It is of course possible to reduce the slag to a powder by crushing and grinding and such a step is indicated on the process chart of Figure 6. It has been found, however, that it is a great convenience and economy to use a slag which is'self-disintegrating, due to the volume changes which the alkaline earth silicates undergo when slowly cooled. The slag comprising: 7

. Per cent CEO Cal 40 S10: 15

is self-disintegrating, as it breaks up into a line powder when cooled slowly to room temperature.

1 The following slag is also self-disintegrating as are also the slags having compositions between those of the two examples just given. disintegration of desulphurizing slags has been noticed in the case of the high lime-fluorspar slags used in the electric steel furnace.

However the slag may be reduced to a powder, whether by crushing and grinding or by selfdisintegration, it is next mixed with water 'to form a'slurry. The slurry is then subjected to the action of carbon dioxide gas, and undergoes the following reactions CaS+H2O+CO2 CaCOs+I-I:S 14) The hydrogen sulphide liberated may then react CaSH-I'BS- CMSH) 2 (15) Seli- Furthertreatment with carbon dioxide as. the reaction Any suitable source oi"carbon dioxide may be employed. As a source of carbon dioxide which is very convenient and economical at an iron or steel plant, it is preferred to use the products oi combustion issuing from furnaces employing blast furnace gas as fuel. For example, the

products oi combustion from the hot blast stoves stantially cooled before being introduced into the slurry, to prevent diiiicultythrough the production of steam in the reviviflcation system.

The products 01' combustion are preferably blown or bubbled through the slurry, in cast iron vessels such as are shown in Figure 7. Each of the vessels II, 55, 58, 51, 58, 58 and 50 is a separate cylindrical tankabout 3 feet (0.9 meter) in diameter and 15 feet (4.6 meters) tall.

It is preferable to use about seven such tanks in conjunction, the exit gases from the tank receiving the gases rich in carbon dioxide passing through a series of other tanks to eiiect the desired reactions.

Each tank has a nitrogen cycle and a hydrogen sulphide cyle. During the operation oi' a given tank as the first tank in the train, the exit gases from this tank are initially nitrogen, which is in ert, excess carbon dioxide and hydrogen sulphide produced by reactions (14) and (16). These gases are passed into other tanks where both hydrogen sulphide and carbon dioxide are absorbed, until the iinal exit gas is substantially nothing but nitrogen, which can'be exhausted.

As the gas treatment continues, a stage is reached in one 01 the tanks in which the exit gases are rich in hydrogen sulphide and low in carbon dioxide. Such gases may be withdrawn and utilized for their sulphur content.

After sufllcient carbon dioxide has been absorbed in a particular tank to complete reactions (14) (15) and (16) the slurry may be withdrawn from thistank and the gas rich in carbon dioxide may be applied to another'tank. The slurry withdrawn from the first tank will be found to contain less than 1% of sulphur.

At any time, gas richin carbon dioxide enters the tanks of Figure 7 through the inlet pipe ll and the exit gases leave through the outlet pipe 82. An inlet header 53 runs across all of the tanks and may be used to carry gas rich in carbon dioxide to any of them.

Each tank is equipped with a down-flow pipe N, 85, 86, 61, 88, 59 or 10, extending from the inlet header 68 to the bottom of the tank. Each tank also has at its top an up-fiow pipe H, 12, I3, II, II, II or II, which joins the inlet header 63. From the up-ilow pipe of each tank to the down-flow pipe of the next tank is a cross-connection I8, I9, 8|), 81, 82, 83 or 84 (the crossconnection 84 is a long pipe extending across the back of the tanks in Figure 7). The various elbows and Ts are provided with suitable capflanges to permit-ready access to the pipes.

Numerous valves are placed to permit change in the direction of gas flow. Between-the juncare not tion with-each down-flow pipe and the Junction with the next up-flow pipe,.the'inlet header 8! has a valve 85, ll, 81, 88, 88, 9|) or'li. I

Between its Junction with the inlet header l8 and its junction with its cross-connection, each down-flow pipe has a valve 82, 83, u, 95, 88, 81 or 88. The inlet ll liesbetween the valves 85 and l2. 1

/ Between its junction with the inlet header 83 and its Junction with its cross-connection, each lip-flow pipe has a valve so, I00, III, I02, I03, I or I85. Eachrross-comiection 18, 18,-",

a M. 82 or II has a valve I", "1,108, I", III) or l|l, while the cross-connection II has two valves III and Ill. Each tank is connected with the outlet through a valve H4, H5, H8, H1, H8, lll'or ill.

- Ii'itbeassumed that the tank "isoutofservice, having its revivifled slag removed, for example by taking oil-' a cap-flange at the bottom, permitting air to enter at the top, and allowing the slurry to flow into any suitable drainage system, the tanksmay operate as follows. Gas high in carbon dioxide entering through the inlet 6| takes a route through valve 82 and down-flow pipe 8| into the bottom of tank 54 and bubbles up through tank 54. 1

A mixture 'of carbon dioxide, hydrogen sulphide and nitrogen may issue from tank 54. This gas is passed by up-flow pipe ll, cross-connection 18, including valve I08 in open position, and down-flow pipe 85 into the bottom of tank 55.

Most of the carbon dioxide and hydrogen sulphide are absorbed in tank 55, but the gas is passed through up-flow pipe 12, crossconnection 7 19, including valve I81 in open position, and

down-flow pipe 86 into the bottom of tank 58.

the nitrogen cycle this eiiluent is allowed escape. l

As the blowing of tank 5| continues, more hydrogen sulphide passes over into tanks 55 and 58, and eventually the quantity of hydrogen sulphide in the eflluent gas from tank 55 becomes appreciable. At this point, the eilluent gas is led through tank 51 by closing valve H6 and directing the gas through up flow pipe 1!, crossconnection 8|) (valve I08 in open position) and down-flow pipe 61 into the bottom 01' tank 51, and then by outlet valve lll into outlet 62. With increase in the hydrogen sulphide content of the eilluent gas from tank 51, it may be desirable to i add tank 58 to the train by closing outlet valve Ill and opening cross-connection valve ")9 and outlet valve H8. i

As the blowing of tank 54 proceeds ,iurther, the effluent gas from tank 56 (which is being carried to tanks 51 and 58) reaches such a high concentration of hydrogen sulphide'that it can be utilized to recover the sulphur. This is the end of the nitrogen, cycle and the beginning of the hydrogen sulphide cycle. Tanks 51 and 58 are cut out of the train by opening outlet valve H5 and closing cross-connection valves I l8 and i0! and outlet valve 8. The outlet 52 is connected to suitable storage or recovery mechanism. As theblowing oi tank 54 continues still further, the hydrogen sulphide content of the eiliuent gas from tank 56 decreases and carbon dioxide begins to come over into the eflluent gas from tank 55.. This is the end of the hydrogen sulphide cycle and the beginning of another nitrogen cycle. Othentanks are then put back in train, for example by opening valves I08, I09 and H8, and closing valve H6. Finally the contents of tank 54 are completely revivified and gas rich in carbon dioxide is led to tank 55 by closing valves 92 and I06 and opening valves 85 and 93. Tank 54 can now be emptied and refilled with slurry to be revivified. At the proper times in the cycle for tank 55, tank 59 is added to the train. In general, other valves not mentioned during the above discussion are kept closed until it is necessary to open them when other tanks are in service.

The above discussion is merely illustrative of one' manner of using the tanks, and it is not intended to limit the disclosure, as other apparatus may be employed or this apparatus may be used in other ways.

,The revivified slag can be removed from the tank in which the process is completed, filter pressed or run through a Dorr thickener or similar apparatus, dried and used for further desulphurization of additional charges of pig iron. The drying may be done in a rotary drier if desired and the charge issuing from the drier may attain temperatures approximating 1000 C. At these temperatures, calcium carbonate is changed to calcium oxide. This hot discharge material can be placed in suitable containers to conserve its heat content and used as a desulphurizing slag for more pig iron.

It will be evident that the compounds used to revivify the slag in accordance with the above processes, namely water and carbon dioxide, are the very substances whose presence in substantial quantities is undesirable during desuiphurization and which are preferably excluded from the desulphurizing vessel.

The point at which the slag is desulphurized will depend, of course, upon the exact process used. If the desulphurization is to be accomplished in stepwise manner, each desulphurizing slag is used twice, as a second slag on one charge and a first slag on the next charge, before it is revivified. It would of course be possible to use the same slag three or a greater number of times before revivification, but this would complicate the process. In any case, when the sulphur content of the slag reaches a predetermined value, the slag is separated from the molten pig iron and revivified.

The gas rich in hydrogen sulphide obtained from revivification may be used for its sulphur content. For example, it may be burned to sulphur dioxide by mixing it with sufiicient air, and

I the sulphur dioxide may be made into sulphuric acid or other suitable compounds. Or, in the presence of a suitable catalyst, the hydrogen sulphide may be burned directly to sulphur trioxide. The hydrogen sulphide may also be converted into elemental sulphur by combustion with the required amount of air in accordance with the following reaction:

Whether the sulphur be changed to sulphuric acid, recovered as elemental sulphur or in some other form, the value of the resulting product may be credited against the cost of revivifying the slag, and may in some cases more than pay for the cost of revivifieation.

Figure 6 shows, by way of example, a series of steps which may be employed in the process. The ladle shown at the top of the chart may suitably contain 100 tons of molten pig iron of say 0.26% sulphur content. Into the ladle is to form sulphur dioxide.

charged about 2.5 tons of a first lime-fluorspar desulphurizing slag containing about 2.40% sulphur. This high sulphur content is due to previous desulphurizing use of the slag subsequent to reviviflcation. After suitable contact between the molten pig iron and the first desulphurizing slag in the ladle, the slag is separated from the pig iron. The first slag nowcontains, for example, 10.8 sulphur.

The slag is now ground to say 50 mesh per linear inch or allowed to self-disintegrate by slow cooling and pre-selection or the proper composition. The ground slag is then mixed with water to form a slurry and treated with a gas high in carbon dioxide, by which the slag is reviviiied, and hydrogen sulphide gas driven off. The hydrogen sulphide gas may be burned in a suitable furnace to liberate sulphur, or, in the alternative, The reviviiied slurry is next dewatered and calcined to produce a revivified slag containing, say 1% of sulphur.

The revivified slag is then supplied to a subsequent charge in the ladle and there used as a second slag for treating pig iron which was initially desulphurized with a previous slag. The pig iron before treatment with the second slag may have a sulphur content of say 0.05% sul- =phur,but after treatment with the second slag its sulphur content will drop to perhaps 0.015% sulphur. The sulphur content of the slag, in the meantime, will increase from about 1% to about 2.40%. This second slag is then available for use as a first slag to treat a new charge of pig iron.

Through both of the desulphurizing treatments, the ladle is maintained under reducing conditions by the presence of carbon and the positive exclusion of the atmosphere and combustion gases high in carbon dioxide and water vapor. The mixture of oxides of carbon present in the ladle is gredominantly reducing due to the excess of car- It is contemplated that there will be certain lossesof slag during the desulphurizing process, and that certain small amounts of impurities may be picked up, both of which features'will necessitate additions of fresh slag-making materialsfrom time to time. By the use of proper refractories in the ladle car, by the maintenance of re.- ducing conditions when the slag is in contact with the molten pig iron, and by suitable treatment with carbon dioxide followed by proper processing of the slurry produced in that operation, the fresh slag-making materials required can be kept to a minimumin order to efiect, to the fullest extent, the economies made possible by repeated use of the slag.

One process of revivifying the slag by driving off the sulphur as a volatile sulphur component in the form of hydrogen sulphide has just been discussed. This process may be varied by treating the powdered slag at higher temperature with steam and carbon dioxide gas, instead of carry-' ing out the process by treating a slurry at moderate temperature.

There are a number of other processes by which reviviflcation may be accomplished. For example, as indicated in Figure 6a, sulphur may be removed from the slag directly as sulphur dioxide gas, by maintaining the high-sulphur slag molten and at high temperature after it is removed from the ladle car and blowing air or oxygen through the slag. In this way sulphur dioxide will form readily and the sulphur content of the slag may be effectively reduced.

' Another process of revivifyi z the slag,

shown in Figure 6b, is to allow it to cool, and reduce it toa fine powder, as by self-disintegration or by crushing and grinding. A portion of the slag is then roasted in a conventional furnace used for roasting sulphides, desirably to a temperature of about 1000 C. Calcium sulphide is thereby changed to calcium sulphate and some of the sulphur is idrivenioff as sulphur dioxide.

The slag containing calcium sulphate is then mixed with a theoretical quantity of unroasted slag and melted to cause calcium sulphide and calcium sulphate to reac CES+3C8SO4=4CBO+4BO2 V (1a) This reaction is carried out with the slag in molten condition at about 1200 C. to 1400 C. In this way the greater part of the sulphur from the sulphur-rich slag is driven off as sulphur dioxide, and may be used for manufacturing sulphuric acid or in other suitable manner.

A serious difllculty with both of the last-mentioned processes for revivifying the desulphurizing slag is that they require that the slag be maintained molten under oxidizing conditions. Slags high in lime and calcium fluoride are rather corrosive to refractories other than carbon and.

while carbon may be used in the ladle car under reducing conditions, it may not be used in operations involving the volatilization of sulphur dioxide where oxidizingconditions are necessary.

It is therefore obligatory to have recourse to magnesite and similar refractories, and magnesite brick may be attacked by the slag.

Another process for revivifying the slag, as indicated in Figure 6c, involves the conversion of the sulphur in the slag to a metal sulphide capable of forming a molten phase immiscible with the molten slag, known as a matte. Cop- 1 per is a suitable matte-forming metal; nickel might also be used. A sulphide of the type RxS reacts with a matte-forming metal thus R,S+2Cu+%O,vR,O-I-2Cn S (l9) Molten slag is brought into contact with molten copper preferably at about 1200 C., and sumcient oxidation is brought about, as for example by blowing air mildly into the copper. Cu-prous sulphide forms and entersthe layer of copper below the slag. The revivifled slag may subsequently be separated from the matte and used for desulphurizing a further charge of pig iron.

The sulphur picked up by the copper may be eliminated by intensive oxidation of the matte through vigorous blowing with air, as for example in the conventional copper matte converter:

nomic'al in many instances.

Numerous examples have been given'herein to aid in practicing the invention. It is not intended, however, to limit the disclosure by reason of the inclusion of these examples except where limitations are included in the claims or indi cated by the specification to be essential.

Where reference is made herein to atmospheric pressure, it is intended to designate the prevailing atmospheric pressure. I

It will be evident that, by exposing the pig iron to the desulphurizing slag under a pressure below atmospheric pressure for a sufficient duration, a higher ratio between the sulphur in the slag and the sulphur in the pig iron can be built up than would be obtained with the same slag under atmospheric .pressure. duced pressure the same desulphurization may be obtained with a smaller quantity of slag than under atmospheric pressure, or under reduced pressure more complete desulphurization may be obtained than under atmospheric pressure with the same quantity of slag, or desulphurization may be obtained under reduced pressure with a slag which would not desulphurize or might even sulphurize under atmospheric pressure.

duced pressure will build up a higher ratio of 7 sulphur in the slag to sulphur in the pig iron in about the same time that it would build up a lower and maximum ratio under atmospheric pressure. I

In view of my invention and disclosure, vari- Thus under reations and modifications to meet individual whim or'particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the process shown, and I, therefore, claim all such in so far as they fall within the reasonable spirit andjscope of my invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is: l

1. The process of producing desulphurized pig iron, which comprises smelting iron ore in a coke blast furnace under conditions of temperature and basicity of the slag suitable to produce pig iron at lower cost, but with a higher sulphur content in the iron than that ultimately desired, transferring the pig iron from the blast furnace, and subsequently treating the molten pig iron under reducing conditions with a basic slag capable of removing sulphur from the pig iron while subjecting the slag and pig iron to a pressure less than atmospheric pressure to facilitate the desulphurizing reaction.

2. The process of producing desulphurized pig iron, which comprises smelting iron ore to produce pig iron containing more sulphur than the content ultimately desired, treating the pig iron as a wholly separateoperation subsequent to production of the pig iron under reducing conditions witha basic slag containing more than 30% of oxides of the type RIO, where R is a metal of the group consisting of alkaline earth metals and alkali metals and :c is a numeral determined by the valence of the metal, and subjecting the slag and pig iron during the treatment to a pressure less than atmospheric pressure to facilitate the desulphurizing reaction.

3. The process of desulphurizing pig iron,.

which comprises smelting iron ore in a coke blast furnace to make pig iron under conditions which produce a substantially higher sulphur content than that ultimately desired-transferring the pig iron from the coke blast furnace, subsequently subjecting the molten pig iron under reducin conditions to a basic slag containing a substantial quantity of lime and fluorspar and subjecting the slag and pig iron to a pressure less than atmospheric pressure to facilitate the desulphurizing reaction.

4. In the process of desulphurizing iron, the step which sconsists in treating molten iron with a basic desulphurizing slag while subjecting the slag and iron to a pressure less than atmospheric pressure, for a duration sufficient to build up a higher ratio between the sulphur in the iron and the sulphur in the slag than would be obtained with the same slag under atmospheric pressure and under substantially equilibrium conditions.

5. The process of producing desulphurized pig iron, which comprises smelting iron ore in a coke blast furnace to make pig iron without regard to the content of sulphur in the resulting pig iron, transferring the pig iron from the blast furnace, treating the pig iron in molten condition with successive additions of a basic slag capable of removing sulphur from the pig iron, subsequent to transfer of the pigiron from the coke blast furnace, while protecting the slag from oxidizing gases, removing each addition before the next is added, and subjecting the slag and pig iron to a pressure less than atmospheric pressure during the treatment to facilitate the desulphurizing reaction.

6. The process of producing desulphurized pig iron, which comprises smelting iron ore in a coke blast furnace under conditions of temperature and basicity of the slag suitable to produce pig iron at lower cost, but with a sulphur content in excess of 0.10%, transferring the pig iron from the blast furnace, and subsequently treating the molten pig iron under reducing conditions with a total of not more than 8% of its weight of basic slag capable of removing sulphur from the pig iron, while subjecting the slag and pig iron to a pressure less than atmospheric pressure, thereby lowering the sulphur content of the pig iron to less than 0.05% and building up a higher ratio between the sulphur in the slag and the sulphur in the pig iron than would be obtained with the same slag under atmospheric pressure and under substantially equilibrium conditions.

7. The process of desulphurizing pig iron, which comprises treating the molten pig iron outside of the blast furnace with a total'of not I more than 8% of its weight of basic desulphurizing slag under reducing conditions and under a pressure less than atmospheric pressure, thereby building up a higher ratio between the sulphur in the slag and the sulphur in the pig iron than would be obtained with the same slag under atmospheric pressure and under substantially equilibrium conditions.

8. The process of producing desulphurized pig iron, which comprises smelting iron ore in a coke blast furnace to make pig iron under conditions which produce a higher sulphur content than that ultimately desired, transferring the pig iron from the blast furnace, treating the molten pig iron under reducing conditions with basic slag capable of removing sulphur subsequent to transfer from the coke blast furnace while subjecting the slag and pig iron to a pressure less than atmospheric pressure and agitating the molten reactants during desulphurization to facilitate the desulphurizing reaction.

9. The process of desulphurizing pig iron, which comprises treating a charge of pig iron in molten condition with a basic slag which removes sulphur from the pig iron while subjecting the slag and pig iron to a pressure less than atmospheric pressure, separating the slag from the .molten pig iron, treating the slag to lower its sulphur con tent while the slag is out of contact with molten pig iron. f

10. The process of desulphurizing molten pig iron produced in a coke blast furnace, which comprises treating molten pig iron external to the coke blast furnace with a basic slag containing calcium oxide and calcium fluoride while subjecting the slag and pig iron to a pressure less than atmospheric pressure, separating the sulphurbearing slag from the desulphurized pig iron, treating the sulphur-bearing slag to lower its sulphur content and reusing the slag for desulphurizing a. further charge of pig iron.

11. The process of desulphurizing pig iron, which comprises treating a charge of pig iron in molten condition in a carbon lining with a basic slag which removes sulphur while subjecting the slag and pig iron-to a pressure less than the prevailing atmospheric pressure, separating the slag from the molten pig iron, treating the slag to lower its sulphur content while the slag is out of contact with the molten pig iron and out of contact with the carbon lining and reusing the slag to desulphurize a further charge of pig iron.

12. The process of desulphurizing pig iron, which comprises successively subjecting it in molten condition to basic desulphurizing slags of progressively decreasing sulphur content under reduced pressure and regenerating each of the slags when its sulphur content attains a predetermined maximum after use a plurality of times.

13. The process of desulphurizing molten pig iron by slags, which comprises treating molten pig iron at a pressure below atmospheric pressure with a first basic slag which has previously been used since revivification to desulphurize other molten pig iron but which is still capable of removing sulphur, building up a higher ratio between the -sulphur in the slag and the sulphur in the pig iron than would be obtained with the same slag under atmospheric pressure and under substantially equilibrium conditions, separating the first slag from the molten pig iron, revivifying the first slag by removing sulphur from it, treating the molten pig iron at a pressure below atmospheric pressure with a second basic slag which has not been-previously used since revivification to desulphurize molten pig iron and which is capable of removing more sulphur, separating the second slag from the pig iron and using the second slag for an initial desulphurizing treatment of a further charge of pig iron.

14. The process of producing desulphurized pig iron, which comprises smelting iron ore in a coke blast furnace to make pig iron under conditions which produce a substantially higher sulphur content than that desired, transferring the pig iron from the coke blast furnace and, subsequent to transfer, treating the pig iron with a. basic desulphurizing slag which is liquid at 1200 C. while maintaining a noncontaminating reducing gas at a pressure less than atmospheric in contact with the pig iron and slag to facilitate th desulphurizing reaction.

15. The process of producing desulphurized pig iron, which comprises smelting iron ore in a coke blast furnace to make pig iron under conditions which produce a. substantially higher sulphur content than that desired, transferring the pig iron from the coke blast furnace and, subsequent to transfer, treating the pig iron with a basic desulphurizing slag while maintaining carbon monoxide at a pressure less than atmospheric in contact with the pig iron and slag to facilitate the desulphurizing reaction.

16. The process of desulphurizing pig iron, which comprises tapping pig iron from a blast furnace, tapping blast furnace slag from a blast furnace and subsequently treating the molten pig iron with the molten blast furnace slag while maintaining a pressure substantially below atmospheric pressure.

17. The process of desuiphurizing pig iron, which comprises tapping pig iron from a blast furnace, tapping blast furnace slag from a blast furnace and subsequently treating the molten pig iron with the molten blast furnace slag in asulphur in the slag to the percentage of sulphur in the pig iron in excess of :1 is attained.

19. The process of producing desulphurized pig iron, which comprises treating molten pig iron under reducing conditions with a basic desulphurizing slag under a pressure below 150 millimeters of mercury, whereby a ratio of the percentage of sulphur in the slag to the percentage of sulphur in the pig iron in'excess of 200:1 is attained.

.20. In the manufacture of steel, the process which comprises smelting iron ore in a coke blast furnace under conditions as to temperature characteristic of a charcoal blast furnace and with a blast furnace slag containing more silica than lime plus magnesia, thereby producing pig iron 1 CERTIFICATE Patent N'O 2, ,067

RUSSELL excessively high in sulphur, and desulphurizing the molten pig iron subsequent to removal from the coke blast furnace and prior tocharging into the steel-making furnace by treatment with a basic desulphurizing slag under reducing conditions while subjecting the slag and pig iron to a pressure less than atmospheric pressure to facilitate the desulphurizing react-ion.

21. The process of producing steel, which comprises smelting iron ore under conditions which produce pig iron having a substantially higher sulphur content than that ultimately-desired, removing'the sulphur from the pig iron in molten condition by a basic slag under a pressure less than atmospheric pressure in the presence of carbon in substantial excess of that which will dissolve in the iron and in the absence of oxidizing gases while it is still pig iron by a wholly inde pendent operation subsequent to the complete production of the pig iron and refining the pig iron to make steel as a wholly independent opera- .tion from the desulphurization.

22. In the production of steel, the art which.

comprises smelting iron ore in a coke blast fur- I nace to make pig iron under conditions which produce a substantially higher sulphur content and pig iron to a pressure less than atmospheric pressure during the treatment.

23. The process of treating a bath of molten iron to free it fromsulphur and to maintain the sulphur freedom, which comprises desulphurizing the bath by basic slag under vacuum conditions and subsequently replacing the vacuum by a gas, at about atmospheric pressure, free from oxygen, carbon dioxide, carbon monoxide, and water vapor, the protecting gas being maintained in place until the slag shall have been removed.

RUSSELL P. HEUER.

or CORRECTION. I

Y I March 1, 1958.

P. HEUER. I

It is hereby certified that error appears in the printed specification.

of the above numbered patent requiring correction as follows: Page 1 .first column, line 26, in the numerator of Equation'9, for "3.9" read 8.2; and that the said Letters Patent should be read with this correction therein Signed and sealed this- 3d. day of Henry van Arsdale Acting Commissioner of' Patents.

that the same may conform to the record of the case in the Patent Office.

m, A. De l938e iron from the coke blast furnace and, subsequent to transfer, treating the pig iron with a basic desulphurizing slag while maintaining carbon monoxide at a pressure less than atmospheric in contact with the pig iron and slag to facilitate the desulphurizing reaction.

16. The process of desulphurizing pig iron, which comprises tapping pig iron from a blast furnace, tapping blast furnace slag from a blast furnace and subsequently treating the molten pig iron with the molten blast furnace slag while maintaining a pressure substantially below atmospheric pressure.

17. The process of desuiphurizing pig iron, which comprises tapping pig iron from a blast furnace, tapping blast furnace slag from a blast furnace and subsequently treating the molten pig iron with the molten blast furnace slag in asulphur in the slag to the percentage of sulphur in the pig iron in excess of :1 is attained.

19. The process of producing desulphurized pig iron, which comprises treating molten pig iron under reducing conditions with a basic desulphurizing slag under a pressure below 150 millimeters of mercury, whereby a ratio of the percentage of sulphur in the slag to the percentage of sulphur in the pig iron in'excess of 200:1 is attained.

.20. In the manufacture of steel, the process which comprises smelting iron ore in a coke blast furnace under conditions as to temperature characteristic of a charcoal blast furnace and with a blast furnace slag containing more silica than lime plus magnesia, thereby producing pig iron 1 CERTIFICATE Patent N'O 2, ,067

RUSSELL excessively high in sulphur, and desulphurizing the molten pig iron subsequent to removal from the coke blast furnace and prior tocharging into the steel-making furnace by treatment with a basic desulphurizing slag under reducing conditions while subjecting the slag and pig iron to a pressure less than atmospheric pressure to facilitate the desulphurizing react-ion.

21. The process of producing steel, which comprises smelting iron ore under conditions which produce pig iron having a substantially higher sulphur content than that ultimately-desired, removing'the sulphur from the pig iron in molten condition by a basic slag under a pressure less than atmospheric pressure in the presence of carbon in substantial excess of that which will dissolve in the iron and in the absence of oxidizing gases while it is still pig iron by a wholly inde pendent operation subsequent to the complete production of the pig iron and refining the pig iron to make steel as a wholly independent opera- .tion from the desulphurization.

22. In the production of steel, the art which.

comprises smelting iron ore in a coke blast fur- I nace to make pig iron under conditions which produce a substantially higher sulphur content and pig iron to a pressure less than atmospheric pressure during the treatment.

23. The process of treating a bath of molten iron to free it fromsulphur and to maintain the sulphur freedom, which comprises desulphurizing the bath by basic slag under vacuum conditions and subsequently replacing the vacuum by a gas, at about atmospheric pressure, free from oxygen, carbon dioxide, carbon monoxide, and water vapor, the protecting gas being maintained in place until the slag shall have been removed.

RUSSELL P. HEUER.

or CORRECTION. I

Y I March 1, 1958.

P. HEUER. I

It is hereby certified that error appears in the printed specification.

of the above numbered patent requiring correction as follows: Page 1 .first column, line 26, in the numerator of Equation'9, for "3.9" read 8.2; and that the said Letters Patent should be read with this correction therein Signed and sealed this- 3d. day of Henry van Arsdale Acting Commissioner of' Patents.

that the same may conform to the record of the case in the Patent Office.

m, A. De l938e 

